1. Field of the Invention
The present invention relates to a focus error signal detection device for an optical information read/write apparatus such as an optical disk device.
2. Description of the Related Art
As a method for detecting a focus error signal in an optical information read/write device according to the related art in this field, there is known Foucault method which has a high detection sensitivity in comparison to the other focus error signal detection methods and enables to reliably detect the error signal even if the angle of the optical axis of the incident light flux is changed.
There are various kinds of device for detecting the focus error signal in accordance with Foucault method.
A first example of the focus error signal detection device utilizing Foucault method is disclosed in Japanese Patent Application Laying Open (KOKAI) No. 1-144235. This example is arranged in such a way that a condenser lens and a beam splitter are combined together to form one unit which lens is disposed for converging the light reflected from the optical information medium on the light receiving element surface and which splitter is disposed on an optical path of the reflection light and divide the flux of the reflection light to two luminous fluxes or more.
A second example of the focus error signal detection device of the related art is disclosed in Japanese Patent Application Laying Open (KOKAI) No. 1-98131. In accordance with the second example, the focus error signal is detected by Foucault method with the use of a diffraction grating having two areas in such a way that considering the change of diffraction angle due to the fluctuation of wavelength and the deformation of the optical spot on the light receiving element caused by the diffraction angle change, the divisional line of the light receiving element is inclined with respect to the direction of diffraction so that the outputs of adjacent light receiving elements are always the same.
A third example of the focus error signal detection device of the related art is disclosed in Japanese Patent Application Laying Open (KOKAI) No. 2-49227. In accordance with the third example, the detection device is arranged in such a way that the reflection light reflected from the optical information recording medium passes through a roof type prism and a triangle prism and after that the reflection light is detected by a pair of light receiving elements, each having a detection area which is divided to two parts wherein the focus error signal is detected in a state where the two elements are disposed in a same plane.
However, with respect to the first example of the related art, it is troublesome to adjust the optical axis of the device at the time of detecting the focus error signal with the use of Foucault method.
Also, with respect to the second embodiment of the related art, the divisional line for dividing the light receiving surface of the light receiving element is inclined by 20 to 30 minutes in order to cancel the influence from the wavelength fluctuation of the diffraction grading element whereas it is necessary to incline the light receiving element by 5 to 10 degrees to adjust the position thereof, which makes it difficult to obtain a reliable adjustment of the device for detecting the focus error signal with the use of Foucault method.
Further, with respect to the third example of the related art, a pair of two-divided light receiving elements are disposed in a same plane by using prisms to obtain a compact structure of the device, which however, either disables to fully adjust the focus error signal or enables to adjust the focus error signal with the use of optical elements of very high accuracy as a result of which the cost of the device becomes high.
As mentioned above, the focus detection method with the use of Foucault method is advantageous in that the detection sensitivity is high and the focus can be detected even the optical axis is dislocated whereas it also has a disadvantage that the focus spot diameter is small so that it becomes hard and troublesome to adjust the position of the photo-detector. Such a troublesome adjustment operation has to be done for each of two focus spots, which necessitates two photo-detectors as a result of which it becomes difficult to obtain a small and compact structure of the device.
To obviate such a problem, a focus control device has been proposed in Japanese Patent Application Laying Open (KOKAI) No. 2-14434, for instance. The focus control device disclosed in the patent document comprises two pairs of parallel plates are combined in an X-shape so that two Foucault spots of semi-circle are generated. However, the proposed device disclosed in the patent document, for example, has disadvantages that it is not easy to arrange the parallel plates that it is hard to obtain a compact structure of the device that it becomes necessary to adjust the rotational position of the photodetectors and that the focus is not reliably detected when the optical axis is dislocated since the optical path lengths of the two paths becomes different from each other when the optical axis is inclined.
Also, Japanese Patent Application Laying Open (KOKAI) No. 2-49227 discloses an optical head which comprises a detection system arranged from a photodetector and a prism combined together and disposed on a same plane.
However, the optical head structure disclosed in the patent document has also disadvantages that it is hard to produce a fully satisfactory prism structure and that it is not possible to adjust the position of the photodetector by Foucault method.